There is found in different electronic circuits and units for supplying working voltages to such circuits a need for safety devices in the form of fuses which will trigger when the current in the circuit protected by the fuse becomes excessively high, for instance due to a short-circuit.
The most common type of fuse is the thermal fuse which normally consists of a glass tube which contains a thin filament or wire which melts or burns when a given current passes through the filament, because the product of filament resistance and current generates heat which, when the current is excessive, melts the filament.
A fuse of this kind will function satisfactorily in the event of a complete short-circuit, whereupon the fuse is triggered relatively quickly. In the event of a short-circuit, the fuse permits a high short-circuit current to pass through which is limited solely by the impedance of the supply source. Such high short-circuiting currents are liable to damage conductors, electric contacts and other electronic devices, and may disturb parallel-supplied electronic devices. 0n the other hand, if the current is high and lies close to or immediately above the rated value of the fuse, it may take considerable time for the fuse to trigger, which in some cases can cause considerable problems due to the overloading of other circuit components. Furthermore, a fuse of this kind can be made either slow or quick. Furthermore, when subjected to loads that are close to its rated value, a fuse of this kind may undergo changes caused by aging, such that after a longer installation period the fuse may trigger without the original rated value of the fuse being exceeded.
Presently voltage supply units often have some form of current limitation which permits maximum power to be taken-out by a circuit. This is often achieved by allowing the current to obtain a given maximum value and then reducing the voltage so that the current will never exceed the current limiting value. When the voltage is reduced, this may render the function of many circuits unsafe because a voltage level becomes critical, which may Jeopardize the application as a whole.
In order to overcome this drawback, it is necessary to monitor voltage in addition to monitoring current, so that the current supply will be broken and a fault indicated when the voltage falls beneath a predetermined value. This results in a particularly complex safety system, which may be more comprehensive than the detail to be monitored or supervised. This is a problem, for instance, in equipment which serves many users and where there is a natural desire that a fault that occurs in respect to one user will not interfere with the other users. An example of this type application is found in a telephone network. For instance, if a subscriber suffers a fault, the fault must in no way influence any of the remaining, possibly hundreds of thousands of subscribers. It is important in applications of this nature that the current can be monitored in the simplest but most effective manner, so as to keep installation costs at a reasonable level. Many similar applications are found within the fields of electronics and data technology where an overload protector is desirable and which is rapid and precise without the protector generating radio disturbances and being too complicated. Another desire is that a protector of this kind need not necessarily be replaced with a new component, but can be readily re-set, either manually or automatically, after eliminating the source of the fault.
U.S. Pat. No. 4,531,083 and U.S. Pat. No. 4,531,084 teach a current regulating circuit for a direct-current mains unit which is intended to charge rechargeable batteries without the use of transformers, capacitors, or coils. The emitters of a first and a second transistor are connected via a fifth current limiting PTC-resistor which is able to function as a fuse. Instead of using a plurality of transistors in a Darlington circuit, MOS field effect transistors can be used. The PTC-resistor is intended to operate at a nominal temperature of 125.degree. C. during a battery charging process, wherein the charging current will decrease by 0.45% with each degree of increased working temperature for this resistor. The resistor is also preferably obtained by metallizing with a material which will enable the resistor to function as a fuse.
German Patent No. 2,533,182 teaches a circuit arrangement for indicating a triggered or blown fuse in a current supply arrangement, wherein an optoswitch is used to indicate to a display device that its fuse has been triggered.
There is thus a need to provide a current limiting device which is both rapid and precise, and in which the power generated under normal operating conditions will be very small.